Cell differentiation requires exquisite precision in the control of gene expression, principally through transcription factors that either activate or repress genes. Perturbations in gene regulation have profound consequences in development and disease, and this is particularly evident with respect to the three muscle types. While each muscle type has a unique transcriptional program linked to target gene expression, there is some overlap in the expression of genes between them during development and disease. Thus, it is of critical importance to elucidate the transcriptional circuitry underlying muscle-restricted gene expression in order to develop novel approaches to redirect programs of gene expression that run askew and to potentially optimize conditions for the differentiation of stem cells into muscle. Smooth muscle cells (SMC), for example, are notoriously flexible in their genetic program of differentiation with documented evidence for both transdifferentiation and phenotypic modulation to more primitive states. Myocardin (Myocd), a potent coactivator of the SRF transcription factor, is highly restricted to SMC, can execute a near complete program of SMC differentiation, and is modulated in disease states. Though Myocd activates a number of SMC contractile genes, there is little knowledge as to its role in eliciting SMC growth suppression or contractile activity. Further, we have yet to define Myocd's full potential in regulating gene expression. Strong preliminary data in this proposal show that Myocd can elicit SMC contractile competence and growth suppression while repressing the skeletal muscle program of differentiation. Thus, we hypothesize that Myocd is a bifunctional regulator of muscle differentiation. We propose three specific aims to test this thesis using innovative methods in genetics. In Aim 1, we will test the hypothesis that Myocd is a sufficient and necessary activator of the SMC contractile phenotype by altering Myocd expression in cells and transgenic animals to determine effects on gene expression, growth, and contractile competence. In Aim 2, we will test the hypothesis that Myocd is a potent repressor for the skeletal muscle program of differentiation using in vitro and in vivo models aimed at illuminating mechanisms underlying this novel function of Myocd. Aim 3 will test the hypothesis that Myocd is expressed in common progenitors for skeletal muscle and SMC utilizing new lineage tracing transgenic mice. Collectively, the planned studies will yield novel insight into Myocd's function as both a mediator of the SMC contractile phenotype and as a repressor for skeletal muscle fate. Such information has enormous implications for modulating gene expression programs in the setting of muscle disease and stem cell differentiation. PUBLIC HEALTH RELEVANCE How is a cell's identity achieved and maintained and what drives a cell to change its characteristics in disease? One way is by regulating genes. Here, we propose studies aimed at one such regulator of genes (myocardin) that has the ability to promote one muscle cell type over another. This information has direct applications to a wide variety of diseases including those of the heart and blood vessels, Alzheimer's disease, and asthma.